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Cacoxenite in Miocene Sediments of the Maryland Coastal Plain

Published online by Cambridge University Press:  02 April 2024

Paul P. Hearn Jr. ,
Lucy McCartan ,
David R. Soller ,
M. Dennis Krohn  and
Virginia M. Gonzalez
Paul P. Hearn Jr.
Affiliation:
U.S. Geological Survey, Reston, Virginia 22092
Lucy McCartan
Affiliation:
U.S. Geological Survey, Reston, Virginia 22092
David R. Soller
Affiliation:
U.S. Geological Survey, Reston, Virginia 22092
M. Dennis Krohn
Affiliation:
U.S. Geological Survey, Reston, Virginia 22092
Virginia M. Gonzalez
Affiliation:
U.S. Geological Survey, Reston, Virginia 22092
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Abstract

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Cacoxenite having the composition (Al4.0Fe22.5O7.1(OH)14.3(PO4)17(H2O)23.7)·50.3H2O was identified in a bed of mature quartz sand in the Miocene Calvert Formation near Popes Creek, Maryland. This is the first reported occurrence of this mineral in Atlantic Coastal Plain sediments north of Florida. The cacoxenite occurs as silt-size to sand-size grains, both as irregularly shaped aggregates and as radiating arrays of delicate acicular crystals. The presence of discrete cores and overgrowths in some grains indicates at least two generations of crystal growth. Electron microprobe analyses reveal excess Si and Al (relative to the ideal composition), which is believed to reflect ultra-fine clay particles within the cacoxenite grains. Admixed clays probably served as a substrate for the formation of ferric oxyhydroxides, which were subsequently converted to cacoxenite through the addition of dissolved phosphorus.

Keywords

CacoxeniteGenesisIron oxyhydroxidesOvergrowthsPhosphorus
Type
Research Article
Information
Clays and Clay Minerals , Volume 36 , Issue 5 , October 1988 , pp. 419 - 424
Copyright
Copyright © 1988, The Clay Minerals Society

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